OP0255 IGG4-RELATED DISEASE ACTIVITY ASSESSMENT INFLUENCED BY USE OF 18F-FDG PET/CT: DATA FROM A NORWEGIAN COHORT
Authors: J. Vikse, Ø. Midtvedt, Ø. Molberg, B. T. Svanes Fevang, Ø. Palm, T. Garen, K. B. Norheim, M. Revheim, K. Johnsrud, H. Fretheim, M. Wallenius, G. Bakland, A. M. Hoffmann-Vold
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> The IgG4-related disease (IgG4-RD) responder index (RI) is a validated tool to assess disease activity in IgG4-RD [1]. The RI reflects symptoms attributable to active IgG4-RD as well as relevant findings from the physical examination, imaging and laboratory evaluations, with higher scores reflecting more active disease. Multiorgan involvement and higher RI scores were identified as risk factors for relapse [2], indicating a need for more aggressive treatment. Whole-body <sup>18</sup>F-FDG PET/CT is a sensitive technique for detecting foci of active inflammation, but its use is limited by cost and availability. Furthermore, whether <sup>18</sup>F-FDG PET/CT detects additional organ manifestations which influence RI and treatment decisions, is largely unknown. <h3>Objectives</h3> To compare performance of a pure clinical RI model (c-RI) and a c-RI with addition of <sup>18</sup>F-FDG PET/CT (c-PET-RI) in a well-characterized Norwegian IgG4-RD cohort. <h3>Methods</h3> Adult patients with IgG4-RD seen at the Oslo University Hospital were included if they had performed both <sup>18</sup>F-FDG PET/CT and a comprehensive assessment by a rheumatologist within a four-week interval. In each patient, we calculated c-RI from symptoms, clinical examination, and laboratory studies, and c-PET-RI from the c-RI plus the score from pathological <sup>18</sup>F-FDG organ uptake on the whole-body PET. Both indices were calculated using data from NOSVAR research registry and electronic medical records. We defined IgG4-RD disease activity as low if RI was ≤ 3 and high if RI was &gt; 3 points. We assessed associations of elevated CRP and serum IgG4 (s-IgG4) with reclassification (discrepancy change in disease activity scores (c-PET-RI – c-RI)) by applying logistic regression with odds ratio (OR) and 95% CI to identify patients in which performing <sup>18</sup>F-FDG PET/CT is likely to change treatment decisions. <h3>Results</h3> The study cohort included 53 IgG4-RD patients, of whom 30 had c-RI ≤ 3 points, consistent with low disease activity. In 15/30 patients (50%) with c-RI ≤ 3, the corresponding c-PET-RI was &gt; 3 points, consistent with high disease activity. In these 15 patients, the mean increase in RI points from the <sup>18</sup>F-FDG PET/CT was 6.4 (range 2-12). We found that 14 of these 15 patients (93%) had elevated s-IgG4 and 6 (40%) had elevated CRP. In logistic regression analysis, elevated s-IgG4, but not CRP, was strongly associated with reclassification to high disease activity after<sup>18</sup>F-FDG PET/CT (OR 16.0, 95% CI 1.7-154.6, p = 0.017). Of the 9 patients with c-RI ≤ 3 and normal s-IgG4, only one (11%) was reclassified to high disease activity after <sup>18</sup>F-FDG PET/CT. <h3>Conclusion</h3> In IgG4-RD patients with low disease activity by clinical and laboratory assessment (c-RI), elevated s-IgG4 is associated with detection of asymptomatic organ involvement by <sup>18</sup>F-FDG PET/CT (c-PET-RI). Hence, evaluation with <sup>18</sup>F-FDG PET/CT should be considered in patients with c-RI ≤ 3 points and elevated s-IgG4 to tailor management. In contrast, in patients with c-RI ≤ 3 points and normal s-IgG4, the added yield of <sup>18</sup>F-FDG PET/CT appears to be low. <h3>References</h3> [1]Wallace ZS et al. Arthritis Care Res (Hoboken). 2018;70(11):1671-1678 [2]Zongfei J et al. Arthritis Research &amp; Therapy. 2022;24(1):106. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> Jens Vikse Speakers bureau: Boehringer-Ingelheim, Novartis, Consultant of: Jupiter Life Science, Novartis, Øyvind Midtvedt: None declared, Øyvind Molberg: None declared, Bjørg Tilde Svanes Fevang Speakers bureau: UCB, Consultant of: Lilly, Øyvind Palm: None declared, Torhild Garen: None declared, Katrine Brække Norheim: None declared, Mona Revheim: None declared, Kjersti Johnsrud: None declared, Håvard Fretheim Speakers bureau: Boehringer Ingelheim, Consultant of: Bayer, Grant/research support from: GSK/Actelion, Marianne Wallenius: None declared, Gunnstein Bakland Consultant of: UCB, Anna-Maria Hoffmann-Vold Speakers bureau: Boehringer Ingelheim, Jannsen, Medscape, Merck Sharp &amp; Dohme and Roche, Consultant of: ARXX, Boehringer Ingelheim, Genentech, Jannsen, Medscape, Merck Sharp &amp; Dohme and Roche, Grant/research support from: Boehringer Ingelheim, Jannsen.